Optically addressed spatial light modulator (OASLM) with dielectric mirror comprising layers of amorphous hydrogenated carbon

ABSTRACT

A reflective type liquid crystal optically addressed spatial light modulator has a first transparent substrate ( 1   b ), a first transparent electrode ( 2   b ) formed on the first transparent substrate ( 1   b ) and a photosensitive layer ( 3 ) formed on the first transparent electrode, formed from materials including hydrogenated amorphous silicon carbide (a-Si:C:H). A read-out light-blocking layer ( 4 ) is formed on top of the photosensor layer ( 3 ) and is formed from amorphous hydrogenated carbon (a-C:H). The high reflectance dielectric multilayer mirror ( 5 ) is formed on top of the light-blocking layer ( 4 ) and can be made of alternating the a-Si:C:H layers with a higher refractive index and the a-C:H layers with lower reflective index. The modulator also has a second transparent substrate ( 1   a ), a second transparent electrode ( 2   a ) formed on the second transparent substrate ( 1   a ), and a liquid crystal layer ( 8 ) disposed between the dielectric mirror ( 5 ) and the second transparent electrode ( 2   a ). The invention allows more efficient separation of the input and read lights and increases the read light reflection, resulting in improvements to the input sensitivity, resolution, contrast ratio, and diffraction efficacy.

This invention relates to the field of optically addressed spatial lightmodulators. More particularly, the invention relates to a reflectivetype optically addressed spatial light modulator (OASLM), and a methodfor manufacturing such a device.

Optically addressed spatial light modulators (OASLMs) using liquidcrystals as the modulating material exhibit high speed and highresolution performance and are important in many areas. In terms ofdesign, they are basically a plane sandwich-like structure. Such adevice includes a pair of glass substrates facing each other. Each ofthe substrates is provided with a transparent electrode on the facingside.

On the first transparent electrode, there is formed a photosensitivelayer. A photosensitive layer is an essential component of an OASLM.Light incident upon this photosensitive layer causes its electricalresistance to reduce in comparison to its resistance in zero lightconditions. This resistance change causes a redistribution of anypotentials present across the device. Hydrogenated amorphous silicon(a-Si:H) photosensors, in both photoconductor and photodiodeconfigurations, are used in transmissive and reflective OASLMs. Anamorphous hydrogenated silicon carbide (a-Si:C:H) photosensor differsfrom the a-Si:H photosensor in terms of its higher photosensitivity,dark resistivity, and transmittance for visible light.

On top the photoconductive layer there may be interposed a lightabsorbing layer for more effective optical isolation between the writeand read lights of a reflective OASLM

A dielectric mirror layer is formed next to the light-blocking layer.The dielectric mirror layer is made from multiple layer films withalternating, different, refractive indices. The mirror increases thereflection for the read light, making the device more opticallyefficient.

A pair of orientation films are formed on the dielectric mirror layerand the second transparent electrode. A liquid crystal layer is disposedbetween the orientation films and sealed by use of a sealing member,which also functions as a spacer, and attaches the glass substrates toeach other.

One problem that exists in the OASLMs described above is a lack ofadhesion between the photoconductor layer, the light blocking layer andthe dielectric multilayer mirror. Also, the manufacture of thephotoconductor, the light blocking layer and the dielectric multilayermirror structure is rather difficult and complicated, since theproduction of such a multiple layer structure requires differentprocessing stages for each layer.

According to the present invention there is provided an opticallyaddressed spatial light modulator (OASLM) comprising:

-   -   a first transparent substrate; a first transparent electrode        formed on said first transparent substrate; a photoconductive        layer formed on the first transparent electrode consisting of        hydrogenated amorphous silicon carbide; a light-blocking layer        formed on said photoconductive layer and consisting of        hydrogenated amorphous carbon; a dielectric mirror layer, itself        having a multiple layer structure formed on said light-blocking        layer; a second transparent substrate upon which is formed a        second transparent electrode, and between said second electrode        and the dielectric mirror a liquid crystal layer and orientation        means therefore;    -   characterised in that the light-blocking layer consists of        hydrogenated amorphous carbon.

A light blocking layer made of hydrogenated amorphous carbon as per thecurrent invention provides for a very efficient light barrier. Thisefficient barrier allows the effective image resolution to be increased.It also provides for good isolation of the write light from the readlight, which again improves the optical parameters of the OASLM.

The materials used in the photoconductor, light blocking layer anddielectric mirror are closely related, which allows for the manufactureof the devices to be simplified. The manufacturing stages needed aremuch reduced, as the production of these layers all use very similarprocesses. Therefore the device does not need to be moved betweendiffering processes as much during its manufacture. The simplifiedmanufacturing procedure results in a cheaper and more reliable product.The use of very similar materials for these components also ensures goodadhesion between these layers, again resulting in improved reliability.

The invention incorporates a photoconductive layer formed fromhydrogenated amorphous silicon carbide (a-Si:C:H). This can be formed,for example, by means of a plasma activated CVD (chemical vapourdeposition) method, by use of a gas including silane (Sir), hydrogen(H₂), methane (Ce) or acetylene (C₂H₂). The conductivities in thefinished OASLM of the photoconductive layer in dark and brightconditions can be set to required values by controlling the gas flowvolume ratio of the gases during formation of the layers. Theconductivity of the photoconductive layer in the dark condition is ofthe same order as the conductivity of the liquid crystal layer, which isabout 10⁻¹⁰ to 10⁻¹² S/cm. The impedance of the photoconductive layerand the liquid crystal layer are also of the same order.

The light-blocking layer is formed using hydrogenated amorphous carbon(a-C:H). Such a light-blocking layer has a good characteristic of lightabsorption in the visible region and can be made, for example, fromhydrogenated material including C₂H₂ by means of a plasma activated CVDmethod. The adhesion ability between the light-blocking layer and thea-Si:C:H photoconductive layer is thus improved over the prior art, andthis provides for better image resolution.

The liquid crystal (LC) optically addressed spatial light modulator(OASLM) of the present invention has a high reflectance multilayermirror which can be made of alternating the a-Si:C:H layers, which havea higher refractive index with the a-C:H layers, which have a lowerrefractive index. Such a dielectric mirror layer can be formed, forexample, by means of a plasma activated CVD method using gases includingsilane (SiH₄), hydrogen (H₂), and methane (CH₄), acetylene (C₂H₂) orother hydrogenated gas and liquid material. The refractive index of thea-Si:C:H layer can be set as required by controlling the flow rates orratios of the material gases. We have found that using the abovetechnique provides a particularly high reflectance mirror when sevenlayers are employed, although the present invention is not limited to amirror having this number of layers. Preferably, the conductivity of thedielectric mirror is set between 10⁻⁹ to 10⁻¹² S/cm. The conductivityand light absorption of the a-C:H layers may be set by the rate at whichthey are deposited. A slower rate will result in an increasedconductivity, but will produce a layer that is more absorptive of light.Conversely, increasing the rate of deposition will decreaseconductivity, but produce a more transparent layer. The conductivity ofthe a-Si:C:H layers may be set by controlling the ratios or flow ratesof the material gases.

The current invention allows for the inclusion of a plurality of lightblocking layers. Preferably any additional light blocking layers areincorporated within the layers of the dielectric mirror. The a-C:Hlayers within the mirror can be made partially light blocking, withoutdegrading the performance of the mirror beyond acceptable limits. To dothis, parameters such as flow rate and material gas ratios may beadjusted during the processing of the a-C:H layers, as described above,to reduce the amount of light that can pass through the layer below thatwhich would normally be chosen. Although all layers of the mirror willimpede light to some degree, in this context, a partially light blockinglayer is one in which the amount of absorption is made greater than thatwhich would be optimal in a dielectric mirror. The absorption of each ofthese layers may not be great individually, but when the absorption fromthe totality of light blocking layers is considered, good performancecan be obtained without compromising mirror performance unduly.

Note that in this specification, a reference to a layer of a-Si:C:H ora-C:H having a higher refractive index should be taken to mean it has arefractive index of 2.6 or greater. Reference to a lower refractiveindex should be taken to mean the refractive index is less than 2.6.

When the OASLM of the current invention is written to with, for example,a laser beam, some of the light incident on the a-Si:C:H photoconductivelayer will pass through it and be absorbed by the a-C:H light-blockinglayer. Without this, the write light would tend to be reflected back tothe photoconductive layer by the dielectric mirror and create aneffective reduction in the resolution of the device. The reflectedsignal in this case would effectively be noise. A read light from alight source is inputted and transmitted through the liquid crystallayer. The transmitted light is precisely and efficiently reflected onthe dielectric mirror layer made of alternating a-Si:C:H/a-C:H layersand is transmitted again through the liquid crystal layer. Accordingly,this device can obtain a high diffraction efficacy.

Further, according to the present invention there is provided an opticaldisplay system incorporating an OASLM, the OASLM comprising:

-   -   a first transparent substrate;    -   a first transparent electrode formed on said first transparent        substrate;    -   a photoconductive layer formed on the first transparent        electrode and consisting of hydrogenated amorphous silicon        carbide;    -   a light-blocking layer formed on said photoconductive layer;    -   a dielectric mirror layer, itself having a multiple layer        structure formed on said light-blocking layer;    -   A second transparent substrate upon which is formed a second        transparent electrode, and between said second electrode and the        dielectric mirror a liquid crystal layer and orientation means        therefore;    -   characterised in that the light blocking layer consists of        hydrogenated amorphous carbon.

An OASLM made according to the current invention has a large number ofuses in display systems. OASLMs are commonly employed as the final lightmodulation device in situations where a fine spatial resolution isneeded. High quality projectors, and projectors that are used to displayimages having a three dimensional component, such as holographicdisplays and autostereoscopic displays, are typical display systems thatoften use OASLMs and the current invention will be of particular benefitto such systems.

Optical signal processing systems may use OASLMs as signal processingelements. An OASLM of the current invention may advantageously beemployed in such a system.

Further, according to the present invention, there is provided a methodof manufacturing an OASLM including the steps of:

-   -   forming a first transparent electrode on a first transparent        substrate;    -   forming an hydrogenated amorphous silicon carbide (a-Si:C:H)        photoconductive layer on a first transparent electrode;    -   forming light-blocking layer on said photoconductive layer;    -   forming a dielectric multilayer mirror based on alternating        a-Si:C:H layers with higher refractive index and a-C:H layers        with a lower refractive index, said mirror being formed on said        light-blocking layer;    -   characterised in that the light blocking layer consists of        hydrogenated amorphous carbon (a-C:H).

The use of an a-C:H layer as a light-blocking layer along with amultilayer a-Si:C:H/a-C:H dielectric mirror in an OASLM with an a-Si:C:Hphotosensor is novel technical solution to the problem of opticallydecoupling the write and read light signals. The performancecharacteristics of an OASLM with such a structure is enhanced due to thefollowing specific features of the fabrication technique and theproperties of the a-Si:C:H and a-C:H films:

-   -   the ability to vary the refractive index in a wide range        (1.6–3.7) and, consequently, to decrease the mirror thickness;    -   the ability to control the conductivity of the a-Si:C:H/a-C:H        layers and hence match the electrical characteristics of the        mirror to the other layers in the OASLM optimally;    -   the fabrication of the dielectric multilayer mirror in a        combined single technological cycle together with the        photosensor (a-Si:C:H) and light-blocking (a-C:H) layers

As the current invention uses similar processes for the manufacturing ofthe light blocking layer, the photosensor and the dielectric mirror,production is made much simpler, and reliability of the device isimproved. There will be a much reduced tendency for the layers toseparate, as can happen with the prior art.

The method of manufacture of the present invention preferably uses aChemical Vapour Deposition (CVD) technique to put down the variouslayers. Preferably, a Plasma Activated CVD method is employed, althoughan Electron Spin Resonance CVD technique can also be used. These are allknown methods, and details of their application will not be furtherdiscussed herein.

Further features and advantages will become apparent from the followingand more particular description of the preferred embodiment of theinvention, as illustrated in the accompanying drawings, in which:

FIG. 1 diagrammatically illustrates a cross sectional view of the layerarrangement of a reflective type liquid-crystal spatial light modulatorin accordance with the present invention;

FIG. 2 shows the relationship between the dark current and photocurrentfor the a-Si:C:H/a-C:H structure vs. the dc voltage;

FIG. 3 depicts the spectral dependence of transmittance of thea-Si:C:H/a-C:H structures;

FIG. 4 shows the calculated reflectance spectra of both seven- and ninelayer dielectric mirrors based on the a-Si:C:H/a-C:H in the interval ofthe wavelength 400–1000 nm. The refractive index of the a-Si:C:H layersis equal to 3.7 and the refractive index of a-C:H layers is equal to1.7;

FIG. 5 shows the calculated reflectance spectra of both seven- and ninelayer dielectric mirrors based on the a-Si:C:H/a-C:H layers in theinterval of the wavelength 400–1000 nm. The refractive index of thea-Si:C:H layers is equal to 3.5 and the refractive index of a-C:H layersis equal to 1.6.

FIG. 1 shows a reflective type optically addressed liquid crystalmodulator according to the present invention. In FIG. 1, the OASLM isprovided with glass substrates 1 a and 1 b. Transparent electrodes 2 aand 2 b are disposed on the substrates 1 a and 1 b respectively. Each ofthe transparent electrodes 2 a and 2 b includes ITO (indium tin oxide)transparent conductive films, and is formed by means of a laserablation.

A photoconductive layer 3 is disposed on the transparent electrode 2 b.The photoconductive layer 3 is made of hydrogenated amorphous siliconcarbide (a-Si:C:H) so that the impedance of the photoconductive layer 3changes upon the application of light. The photoconductive layer 3 isformed by means of a plasma CVD (chemical vapour deposition) method.

A light-blocking layer 4 is disposed on the photoconductive layer 3. Thelight-blocking layer 4 is made of hydrogenated amorphous carbon (a-C:H),and has a layer thickness of about 0.5 μm. The light-blocking layer 4prevents the write beam from reflecting at the dielectric mirror layer 5and impinging again on the photoconductive layer 3, which would produceimage degradation.

Since the light blocking layer 4 is made of hydrogenated amorphouscarbon, its light absorbing ability is high. Thus, the effective imageresolution of the modulator can be increased by the existence of thelight absorbing layer 4. In addition, the adhesion between thephotoconductive layer 3 made of hydrogenated amorphous silicon carbideand the light-blocking layer 4 is strong, and thus a detachment of thoselayers 3 and 4 from each other is prevented.

The dielectric mirror layer 5 is situated on the light blocking layer 4.The dielectric mirror layer 5 has a multiple layer structure, withlayers alternating between higher and lower refractive indices. Thehigher refractive index layers are made of hydrogenated amorphoussilicon carbide (a-Si:C:H) and the lower refractive index layers aremade of hydrogenated amorphous carbon (a-C:H). The dielectric mirrorlayer 5 thus constructed has a good reflecting capability and increasesthe image resolution of the modulator. In addition, the dielectricmirror layer 5 thus constructed has an advantage in its simplifiedmanufacturing process, since both of the layers can made in a combinedsingle technological cycle together with the photosensitive (a-Si:C:H)and light-blocking (a-C:H) layers.

Orientation films 6 a and 6 b are applied to the transparent electrode 2a and the dielectric mirror layer 5. The substrates 1 a and 1 b areattached together by a sealing member 7. The liquid crystal layer 8 issituated between the orientation films 6 a and 6 b.

Advantageously, many of the layers making up an OASLM have a high sheetresistance. This is the resistance between two parts of the same layeron the axis of the layer, and if this is high enough, any charge on onepart of the layer is not dissipated across the layer. Too low aresistance will allow signal charge spread and reduce the resolution ofthe device. A high sheet resistance is particularly important in thephotoconductor and light-blocking layers. FIG. 2 illustrates aninfluence of different sheet resistance on the density of dark andphotocurrent of the ITO/a-Si:C:H/a-C:H thin-films structures. The plotsa, b and c in FIG. 2 show the optimally matched voltage dependencies fora light attenuation factor of 100, and photocurrent to dark currentratios of 200 and 1000. Plot b is the dark current plot.

FIG. 3 illustrates the transmittance as a function of wavelength of thea-Si:C:H photoconductor coupled to the a-C:H light-blocking layer withα˜5×10⁴ cm⁻¹ at λ=633 nm. The a-C:H light-blocking layer with thick of0.5 μm effectively isolates green light with λ=550 nm. The transmittancereaches ˜1% for red light, when the thickness of a-C:H layer is near 1μm. The disadvantage with using too thick a a-C:H layer is that itcauses a deterioration of the image spatial resolution in the reflectivetype OASLMs. The 0.5 μm thick a-C:H light-blocking layer and thedielectric mirror having reflectivity equal to 80% incorporated into anOASLM has been used to write and to read a diffraction grating at awavelength of 633 nm. The intensity of read radiation was not found toaffect the photoaddressing of the a-Si:C:H photoconductor.

To choose the optimal design of a multilayer dielectric mirror and tomatch its electrical and optical characteristics to the parameters ofother layers in an OASLM, the reflective spectra as a function ofrefractive index and absorption coefficient of a-Si:C:H and a-C:H layersmust be calculated. Advantageously, the refractive index of an a-Si:C:Hlayer within a dielectric mirror can be set between approximately 3.7 toapproximately 3.3. The a-C:H layers with a lower refractive index ofabout 1.6–1.7 shows a lower absorption coefficient in the visiblespectral range of about 0.01. If the application required a mirror wherethe a-C:H layers were to be used to provide some light blocking functionas described above, then the absorption would be set to a higher figurethan this. In FIG. 4 the calculated reflectance spectra of both sevenand nine layer dielectric mirrors in the interval of the wavelength400–1000 nm are shown. The refractive index of the a-Si:C:H and a-C:Hlayers are equal to 3.7 and 1.7 in this case, accordingly. In FIG. 5 thecalculated reflectance spectra of seven and nine layers dielectricmirrors in the interval of the wavelength 400–1000 nm are shown. In thisthe refractive index of the a-Si:C:H and a-C:H layers are equal to 3.5to 1.6. These parameters were chosen as being particularly practical toincorporate into the normal manufacturing process used when producing ana-Si:C:H/a-C:H dielectric mirror.

The examples of the reflective spectra in FIG. 4 and FIG. 5 are givenfor illustration purposes only and are not meant to limit the scope ofclaims in any way. Note that it is possible to use a layer with thelarger index, however, in this case, its conductivity will be largerthan the dark conductivity of the a-Si:C:H photosensitive layer, whichmay result in the image blurring. The mirrors with seven layers providea peak reflectance of about 95%. The position of the maximum depends onthe various layer thicknesses. Thus, theoretical calculationsdemonstrate that it is possible to fabricate multilayer mirrors witha-Si:C:H and a-C:H layers with the reflectance higher than 95% and witha thickness of about 0.5 μm.

A process for manufacturing a photoconductor/light-blockinglayer/multilayer dielectric mirror structure of the OASLM will beexplained below in sequence.

-   (A) The photoconductive layer 3 is formed on the transparent    electrode 2 b by a plasma CVD method. In this forming process, SiH₄    (silane), H₂ (hydrogen), and at least one of CH₄ (methane) or C₂H₂    (acetylene) are used as material gases. The thickness of the    photoconductive layer 3 is typically about 1.5 μm.-   (B) The light blocking layer 4 is formed on the photoconductive    layer 3 by a plasma CVD method. In this forming process, acetylene    is used as a material gas. Any hydrocarbon gas or liquid material    including methane (CH₄) can be used too. The thickness of the light    absorbing layer 4 is made in the interval from 0.5 to about 1 μm.-   (C) The dielectric mirror layer 5 is formed on the light absorbing    layer 4 by a plasma CVD method. In this forming process, SiH₄, H₂,    and CH₄ are used as material gases to form the alternating a-Si:C:H    layers with a higher refractive index. CH₄ or C₂H₂ gases are used as    material gases for forming the a-C:H layers with lower refractive    index.

1. An optically addressed spatial light modulator (OASLM) comprising: afirst transparent substrate; a first transparent electrode formed onsaid first transparent substrate; a photoconductive layer formed on thefirst transparent electrode comprising hydrogenated amorphous siliconcarbide; a light-blocking layer formed on said photoconductive layer andcomprising hydrogenated amorphous carbon; a dielectric mirror layer,itself having a multiple layer structure formed on said light-blockinglayer; a second transparent substrate upon which is formed a secondtransparent electrode, and between said second electrode and thedielectric mirror a liquid crystal layer and orientation meanstherefore; wherein the light-blocking layer consists of hydrogenatedamorphous carbon and wherein the dielectric mirror is formed fromalternate hydrogenated amorphous silicon carbide (a-Si:C:H) layers andhydrogenated amorphous carbon (a-C:H) layers to form layers of higherand lower refractive index respectively.
 2. An OASLM as claimed in claim1 wherein the a-Si:C:H layers of the dielectric multilayer mirror havethe higher refractive index in the interval from 3.7 to 3.3.
 3. An OASLMas claimed in claim 1 wherein the a-C:H layers of the dielectricmultilayer mirror have the lower refractive index in the interval from1.5 to 1.8.
 4. An OASLM as claimed in claim 1 wherein the a-C:H layersof the dielectric multilayer mirror have the lower refractive index inthe interval from 1.6 to 1.7.
 5. An OASLM as claimed in claim 1 whereinthe conductivities of dielectric mirror layers is in the range from 10⁻⁹to 10⁻¹² Ohm⁻¹cm⁻¹.
 6. An OASLM as claimed in claim 1 wherein theDielectric mirror has seven layers.
 7. An OASLM as claimed in claim 1wherein the light blocking layer has a thickness of between the limitsof 0.4 μm and 0.6 μm.
 8. An OASLM as claimed in claim 1 wherein at leastone of the layers of the dielectric mirror having a lower refractiveindex is so formed to be partially light blocking.
 9. An OASLM asclaimed in claim 1 wherein the refractive index of the dielectric mirrora-Si:C:H layers is 3.5 and the refractive index of the dielectric mirrora-C:H layers is 1.6.
 10. An optical display system incorporating anOASLM, the OASLM comprising: a first transparent substrate; a firsttransparent electrode formed on said first transparent substrate; aphotoconductive layer formed on the first transparent electrode thephotoconductive layer comprising hydrogenated amorphous silicon carbide;a light-blocking layer formed on said photoconductive layer; adielectric mirror layer, itself having a multiple layer structure formedon said light-blocking layer; a second transparent substrate upon whichis formed a second transparent electrode, and between said secondelectrode and the dielectric mirror a liquid crystal layer andorientation means therefore; wherein the light blocking layer consistsof hydrogenated amorphous carbon and wherein the dielectric mirror isformed from alternate hydrogenated amorphous silicon carbide (a-Si:C:H)layers and hydrogenated amorphous carbon (a-C:H) layers to form layersof higher and lower refractive index respectively.
 11. An opticaldisplay system as claimed in claim 10 wherein the display system iscapable of displaying a holographic diffraction grating.
 12. An opticalsignal processing system incorporating an OASLM, the OASLM comprising: afirst transparent substrate; a first transparent electrode formed onsaid first transparent substrate; a photoconductive layer formed onfirst transparent electrode the photoconductive layer comprisinghydrogenated amorphous silicon carbide; a light-blocking layer formed onsaid photoconductive layer the light-blocking layer comprisinghydrogenated amorphous carbon; a dielectric mirror layer, itself havinga multiple layer structure formed on said light-blocking layer; a secondtransparent substrate upon which is formed a second transparentelectrode, and between said second electrode and the dielectric mirror aliquid crystal layer and orientation means therefore; wherein the lightblocking layer consists of hydrogenated amorphous carbon and wherein thedielectric mirror is formed from alternate hydrogenated amorphoussilicon carbide (a-Si:C:H) layers and hydrogenated amorphous carbon(a-C:H) layers to form layers of higher and lower refractive indexrespectively.
 13. An optically addressed spatial light modulator (OASLM)comprising: a first transparent substrate; a first transparent electrodeformed on said first transparent substrate; a photoconductive layerformed on the first transparent electrode, the photoconductive layercomprising hydrogenated amorphous silicon carbide; a light-blockinglayer formed on said photoconductive layer, the light-blocking layercomprising hydrogenated amorphous carbon; a dielectric mirror layer,itself having a multiple layer structure of higher and lower refractiveindex material so formed as to be partially light blocking; and a secondtransparent substrate upon which is formed a second transparentelectrode, and between said second electrode and the dielectric mirror aliquid crystal layer and orientation means therefore wherein thelight-blocking layer includes hydrogenated amorphous carbon.